Restless in temperament and unconventional in method, he plunged into the vortex of German politics to a degree that did not always accord with the traditions of diplomacy.

In December 1884 he became ambassador at St. Petersburg, and almost immediately had to face the alarming situation created by the Russian incursion into Afghanistan, known as the Panjdeh incident.

The Kölnische Zeitung [de] declared in December 1888 that Morier had made use of his position at Darmstadt during the Franco-Prussian War to betray the movements of the German troops to Marshal Bazaine.

The authority for this charge was an alleged declaration made by Bazaine to the German military attaché at Madrid, Adolf von Deines.

Morier's failing health caused him, at his own request, to be appointed Lord Dufferin's successor at Rome in 1891; but it was felt that he could not be spared from St. Petersburg, and there he remained until forced to find a milder climate.